Purpose: The aim of this study is to explore how pedicle screws (PSs) and cortical bone trajectory (CBT) screws differ in fixation strength when implanted in L1-L5 with osteoporosis, providing support for choosing implants and trajectories in spine internal fixation surgeries. Methods: We filtered 30 lumbar segments out from CT images of eight osteoporotic participants and simulated PS or CBT screw implantation in each segment, generating 60 vertebra-screw assembly FE models. To evaluate the fixation effect, we performed a pull-out force test simulation in each model and analyzed the maximal pull-out force, pull-out stiffness, and equivalent stress of vertebrae and screws. Results: The maximal pull-out force of PS and CBT screws in L1-L5 was in the range of 905-1552 (N) and 587-1012 (N), while the pull-out stiffness was in the range of 1990-2617 (N/mm) and 1007-1681 (N/mm). The fixation strength of PS in L4 and L5 was higher (p < 0.05), while in L1-L3 PS and CBT screws are similar (p > 0.05). The maximal stress of vertebrae and screws when PS was pulled at 0.25 ram was larger than that of CBT screws. Conclusions: For patients with moderate osteoporosis, it is recommended to insert PS into L4 and L5 to attain better fixation strength, but vertebrae are more prone to fracture. Consequently, under severe osteoporosis, the implantation of CBT screws should be considered first. Bone cement injection may be necessary to consolidate the screw-vertebrae interface with osteoporosis.